The present invention relates to a process for the removal of sulfur oxides from hot gases containing the same. It particularly relates to a process wherein two gas streams containing sulfur oxides are concurrently contacted with an aqueous sulfur oxide absorbent in a manner such that when they are combined the effluent gas stream is water unsaturated, has a substantially reduced sulfur oxide content and substantially complete utilization of the absorbent is obtained.
Sulfur oxides, principally present as sulfur dioxide, are found in the waste gases discharged from many metal refining and chemical plants, and in the flue gases from power plants generating electricity by the combustion of fossil fuels. In addition, sulfur-containing gases may be formed in the partial combustion or gasification of sulfurcontaining fuels, such as coal or petroleum residua. The control of air pollution resulting from the discharge of sulfur oxides into the atmosphere has become increasingly urgent. An additional incentive for the removal of sulfur oxides from waste gases is the recovery of the sulfur values otherwise lost by discharge to the atmosphere. However, in some instances, such as when the flue gases are from power plants, which based on the combustion of an average coal may contain as much as 3000 ppm sulfur dioxide and 30 ppm sulfur trioxide by volume, the large volumes of these gases relative to the quantity of sulfur which they contain make recovery or removal of the sulfur compounds expensive. Thus, while the possible by-products, such as elemental sulfur and sulfuric acid, that ultimately may be obtained from the recoverable sulfur values have virtually unlimited markets as basic raw materials, they sell for relatively low prices. Consequently, an inexpensive sulfur recovery process is required.
Many processes have been proposed and investigated for the desulfurization of flue gases. For example, dry processes have been proposed in which the sulfur dioxide is removed either by chemical reaction with a solid absorbent or by absorption on its surface followed by oxidation of the adsorbed sulfur dioxide. In U.S. Pat. No. 2,718,453 there is shown a process wherein finely powdered calcium carbonate is blown into a combustion gas to form calcium sulfate or calcium sulfite. In general, a reaction between a solid and a gas is relatively slow and inefficient, being limited by the available surface area of the solid. Also certain of the resultant products do not readily lend themselves to regeneration of the starting material or recovery of the removed sulfur values.
In U.S. Pat. Nos. 3,438,722; 3,438,727; and 3,438,728 sulfur oxide impurities are removed from a hot combustion gas by contacting it at an elevated temperature with a molten salt mixture as the active absorbent. The spent absorbent is then regenerated chemically and recirculated. A disadvantage of this process is that a retrofit to many older existing power plant utility installations presents certain economic disadvantages because of the requirements for modifying the boiler systems of these plants to obtain a flue gas having the required elevated temperature for treatment with a molten salt.
Wet absorption processes are suitable for treating low temperature flue gases or hot gases if a precooling step is included. In a typical wet absorption process, the flue gas is washed with an aqueous slurry of calcium hydroxide or calcium carbonate. This type of process has been utilized in many power plants. In addition, aqueous sodium sulfite or ammonia solutions also have been utilized in wet processes.
A typical wet absorption process is described in U.S. Pat. No. 3,533,748 wherein a waste gas stream containing sulfur oxides is scrubbed with an aqueous solution of a soluble alkali such as sodium carbonate or sodium hydroxide to form sulfite and sulfate in solution. The resulting solution is then evaporated to precipitate solid alkali metal sulfite and sulfate salts, which can be separated from the solution and further processed.
While these wet absorption processes have some advantages, they suffer from a common drawback of producing a liquid effluent containing a large amount of water relative to the sulfur oxide absorbed, which effluent is not amenable to simple high temperature reduction and regeneration. Thus, difficulties arise where economic and efficient recovery of the absorbent and sulfur values is desired. In many instances, the recovery of elemental sulfur, a preferred product, is not economical. Still further, the treated gas stream is saturated with water and if discharged to the atmosphere without reheating will form an objectionable plume. An additional drawback of the wet absorption processes is their requirement of a low temperature, water-saturated feed gas to avoid evaporation of the absorbent. Since most flue gases are available at a temperature of 100.degree. -300.degree. C. it is common practice to install a water quench gas cooler unit ahead of the wet scrubber to precondition the feed gas.
A more recent approach to the removal of sulfur oxides from hot gas streams is shown in U.S. Pat. No. 3,932,587. A hot sulfur oxide containing waste gas is contacted in a spray dryer with an aqueous scrubbing medium containing an absorbent selected from the group consisting of alkali metal carbonates, bicarbonates and mixtures thereof. The scrubbing medium is introduced in a sufficient quantity to provide a resultant reaction product comprising a flowable, dry powder mixture containing from 20-85 wt. % alkali metal sulfite and sulfate; the balance comprising unreacted absorbent. Thereafter, the dry powder product is treated to recover hydrogen sulfide gas as a product and regenerate the alkali metal absorbent for recirculation in the process. A disadvantage of this process is that it requires an active alkali metal absorbent to get high sulfur oxide removal in a single spray drying zone. In addition, it does not provide for the substantially complete utilization of the absorbent concurrently with high sulfur oxide removal.
U.S. Pat. No. 4,197,278 discloses a two-zone method of removing particulates sulfur oxides from a hot gas. The hot gas is contacted in a first spray dryer zone with a selected chemically reactive absorbent for the sulfur oxides and then sequentially contacted with the absorbent in a second fabric filter zone. The conditions of contact in the first spray dryer zone are controlled such that a substantial portion of the sulfur oxide and unreacted absorbent are carried over into a fabric filter zone; it being reported that the unreacted absorbent under such conditions will have an enhanced reactivity. Thus, the unreacted absorbent of enhanced reactivity coats the surface of the fabric filter and reacts with the sulfur oxide contained in the gas passing therethrough. This process provides a dry effluent gas and relatively high sulfur oxide removal. However, it is noted in the examples that high absorbent utilization is not obtained with the less reactive absorbents such as calcium hydroxide.
U.S. Pat. No. 4,198,380 discloses another process for removing sulfer oxides by sequential absorbent of the sulfur oxides in two zones. In each zone the hot gas is contacted with an aqueous scrubbing medium containing an alkali metal absorbent for the sulfur oxides. The aqueous medium is introduced into a spray dryer zone in an amount to produce a dry powder reaction product substantially free of unreacted absorbent and a substantially water-unsaturated gaseous reaction product of reduced sulfur oxide content. The gaseous reaction product of reduced sulfur oxide content is transferred into a wet scrubber zone where it is contacted with a sufficient amount of an aqueous solution of absorbent to provide a gaseous reaction product substantially free of sulfur oxide and an aqueous solution containing alkali metal sulfites, sulfates and unreacted absorbent, which solution is used as at least a part of the aqueous solution fed into the first zone. A disadvantage of this process is that like the wet scrubbing processes, it produces an effluent gas which is water saturated and will produce an objectionable plume if discharged to the atmosphere without reheating. Further, the total quantity of gas to be processed must pass through both a spray dryer-type absorber and a wet scrubber in series which leads to high equipment costs.